I tested datum-label support with the following expression:
(let ((x '(#1=(a b) #1#))) (eq? (car x) (cadr x)))Gauche, MIT, Bigloo, Kawa, Chez, Ikarus/Vikare, Larceny*, Mosh, STklos, FemtoLisp, Sagittarius, Foment, Chibi, Picrin support the syntax and return #t;
Racket, Chicken, Scheme48/scsh, Guile, SISC, SCM, Larceny*, Ypsilon, IronScheme, NexJ, JScheme, KSi, SigScheme, TinyScheme, Scheme 9, RScheme, S7, XLisp, Rep, Schemik, Elk, SXM, Sizzle, Dfsch, Inlab, Oaklisp, Owl Lisp report lexical syntax errors;
Gambit, Shoe, BDC, UMB, Llava return #f for whatever reason.
*Larceny v0.98 normally recognizes the syntax, but reports a lexical error in R6RS mode (as is required by the R6RS).